A comparison of the effects of p(62)-Be and d(16) -Be neutrons in the mouse kidney

Renal damage in the mouse was assessed after irradiation with p(62)-Be neutrons and compared with the renal response to d(16)-Be neutrons. One, 2, 4 and 8 fractions of radiation were given, ore 8 low-dose fractions followed by a 'top-up' dose of d(4)-Be neutrons at the Gray Laboratory. The use of both full course fractionation and the top-up technique in this study allowed ratios of isoeffective neutron dose at the two energies (NDR) to be measured over a wide range of neutron dose per fraction from 0.2 to 9.6 Gy. NDR (neutron dose ratio) is a direct way of specifying the relative RBE's of the two neutron beams. Radiation injury in the kidney was assayed with three methods: clearance of [51Cr] EDTA, reduction in haematocrit, and urine output. The dose-response curves obtaned were resolved best at 31 weeks post-irradiation in these experiments. All three assays give similar results. NDR was roughly constant at n 1.38 over the complete dose range. For equal effects in the kidney, the standard Hammersmith protocol for d(16)-Be neutrons of 17.6 Gy (N + γ dose) given in 12 fractions would require 24.6 Gy in 12 fraction using d(16)-Be beam at Clatterbridge. NDR for renal damage was greater than the NDR obtained previously for mouse skin (1.1-1.21) at all doses, indicating the kidneys would be spared in fractionated treatments with p(62)-Be neutrons compared with d(16)-Be neutrons if radiation doses were given to the same skin tolerance. Examination of the RBE of the two neutron beams relative to 240 k Vp x-rays showed that this reflects a lower RBE compared with skin for p(62)-Be neutrons in the clinical dose-range, and a higher RBE compared with skin for d(16)-Be neutrons. These radiobiological data therefore favour the use of high energy neutron therapy if kidneys are included in the field, but this conclusion should not be extrapolated to other late responding normal tissues for which further experimental data should be accrued. 27 refs.; 6 figs.; 2 tabs